Expression monitoring by hybridization to high density oligonucleotide arrays

ABSTRACT

This invention provides methods of monitoring the expression levels of a multiplicity of genes. The methods involve hybridizing a nucleic acid sample to a high density array of oligonucleotide probes where the high density array contains oligonucleotide probes complementary to subsequences of target nucleic acids in the nucleic acid sample. In one embodiment, the method involves providing a pool of target nucleic acids comprising RNA transcripts of one or more target genes, or nucleic acids derived from the RNA transcripts, hybridizing said pool of nucleic acids to an array of oligonucleotide probes immobilized on surface, where the array comprising more than 100 different oligonucleotides and each different oligonucleotide is localized in a predetermined region of the surface, the density of the different oligonucleotides is greater than about 60 different oligonucleotides per 1 cm 2 , and the olignucleotide probes are complementary to the RNA transcripts or nucleic acids derived from the RNA transcripts; and quantifying the hybridized nucleic acids in the array.

BACKGROUND OF THE INVENTION

[0001] Many disease states are characterized by differences in theexpression levels of various genes either through changes in the copynumber of the genetic DNA or through changes in levels of transcription(e.g. through control of initiation, provision of RNA precursors, RNAprocessing, etc.) of particular genes. For example, losses and gains ofgenetic material play an important role in malignant transformation andprogression. These gains and losses are thought to be “driven” by atleast two kinds of genes. Oncogenes are positive regulators oftumorgenesis, while tumor suppressor genes are negative regulators oftumorgenesis (Marshall, Cell, 64: 313-326 (1991); Weinberg, Science,254: 1138-1146 (1991)). Therefore, one mechanism of activatingunregulated growth is to increase the number of genes coding foroncogene proteins or to increase the level of expression of theseoncogenes (e.g. in response to cellular or environmental changes), andanother is to lose genetic material or to decrease the level ofexpression of genes that code for tumor suppressors. This model issupported by the losses and gains of genetic material associated withglioma progression (Mikkelson et al. J. Cellular Biochm. 46: 3-8(1991)). Thus, changes in the expression (transcription) levels ofparticular genes (e.g. oncogenes or tumor suppressors), serve assignposts for the presence and progression of various cancers.

[0002] Similarly, control of the cell cycle and cell development, aswell as diseases, are characterized by the variations in thetranscription levels of particular genes. Thus, for example, a viralinfection is often characterized by the elevated expression of genes ofthe particular virus. For example, outbreaks of Herpes simplex,Epstein-Barr virus infections (e.g. infectious mononucleosis),cytomegalovirus, Varicella-zoster virus infections, parvovirusinfections, human papillomavirus infections, etc. are all characterizedby elevated expression of various genes present in the respective virus.Detection of elevated expression levels of characteristic viral genesprovides an effective diagnostic of the disease state. In particular,viruses such as herpes simplex, enter quiescent states for periods oftime only to erupt in brief periods of rapid replication. Detection ofexpression levels of characteristic viral genes allows detection of suchactive proliferative (and presumably infective) states.

[0003] Oligonucleotide probes have long been used to detectcomplementary nucleic acid sequences in a nucleic acid of interest (the“target” nucleic acid) and have been used to detect expression ofparticular genes (e.g., a Northern Blot). In some assay formats, theoligonucleotide probe is tethered, i.e., by covalent attachment, to asolid support, and arrays of oligonucleotide probes immobilized on solidsupports have been used to detect specific nucleic acid sequences in atarget nucleic acid. See, e.g., PCT patent publication Nos. WO 89/10977and 89/11548. Others have proposed the use of large numbers ofoligonucleotide probes to provide the complete nucleic acid sequence ofa target nucleic acid but failed to provide an enabling method for usingarrays of immobilized probes for this purpose. See U.S. Pat. Nos.5,202,231 and 5,002,867 and PCT patent publication No. WO 93/17126.

[0004] The use of “traditional” hybridization protocols for monitoringor quantifying gene expression is problematic. For example two or moregene products of approximately the same molecular weight will provedifficult or impossible to distinguish in a Northern blot because theyare not readily separated by electrophoretic methods. Similarly, ashybridization efficiency and cross-reactivity varies with the particularsubsequence (region) of a gene being probed it is difficult to obtain anaccurate and reliable measure of gene expression with one, or even afew, probes to the target gene.

[0005] The development of VLSIPS™ technology provided methods forsynthesizing arrays of many different oligonucleotide probes that occupya very small surface area. See U.S. Pat. No. 5,143,854 and PCT patentpublication No. WO 90/15070. U.S. patent application Ser. No. 082,937,filed Jun. 25, 1993, describes methods for making arrays ofoligonucleotide probes that can be used to provide the complete sequenceof a target nucleic acid and to detect the presence of a nucleic acidcontaining a specific nucleotide sequence.

[0006] Prior to the present invention, however, it was unknown that highdensity oligonucleotide arrays could be used to reliably monitor messagelevels of a multiplicity of preselected genes in the presence of a largeabundance of other (non-target) nucleic acids (e.g., in a cDNA library,DNA reverse transcribed from an mRNA, mRNA used directly or amplified,or polymerized from a DNA template). In addition, the prior art providedno rapid and effective method for identifying a set of oligonucleotideprobes that maximize specific hybridization efficacy while minimizingcross-reactivity nor of using hybridization patterns (in particularhybridization patterns of a multiplicity of oligonucleotide probes inwhich multiple oligonucleotide probes are directed to each targetnucleic acid) for quantification of target nucleic acid concentrations.

SUMMARY OF THE INVENTION

[0007] The present invention is premised, in part, on the discovery thatmicrofabricated arrays of large numbers of different oligonucleotideprobes (DNA chips) may effectively be used to not only detect thepresence or absence of target nucleic acid sequences, but to quantifythe relative abundance of the target sequences in a complex nucleic acidpool. In particular, prior to this invention it was unknown thathybridization to high density probe arrays would permit small variationsin expression levels of a particular gene to be identified andquantified in a complex population of nucleic acids that out number thetarget nucleic acids by 1,000 fold to 1,000,000 fold or more.

[0008] Thus, this invention provides for a method of simultaneouslymonitoring the expression (e.g. detecting and or quantifying theexpression) of a multiplicity of genes. The levels of transcription forvirtually any number of genes may be determined simultaneously.Typically, at least about 10 genes, preferably at least about 100, morepreferably at least about 1000 and most preferably at least about 10,000different genes are assayed at one time.

[0009] The method involves providing a pool of target nucleic acidscomprising mRNA transcripts of one or more of said genes, or nucleicacids derived from the mRNA transcripts; hybridizing the pool of nucleicacids to an array of oligonucleotide probes immobilized on a surface,where the array comprises more than 100 different oligonucleotides, eachdifferent oligonucleotide is localized in a predetermined region of saidsurface, the density of the different oligonucleotides is greater thanabout 60 different oligonucleotides per 1 cm², and the olignucleotideprobes are complementary to the mRNA transcripts or nucleic acidsderived from the mRNA transcripts; and quantifying the hybridizednucleic acids in the array. In a preferred embodiment, the pool oftarget nucleic acids is one in which the concentration of the targetnucleic acids (mRNA transcripts or nucleic acids derived from the mRNAtranscripts) is proportional to the expression levels of genes encodingthose target nucleic acids.

[0010] In a preferred embodiment, the array of oligonucleotide probes isa high density array comprising greater than about 100, preferablygreater than about 1,000 more preferably greater than about 16,000 andmost preferably greater than about 65,000 or 250,000 or even 1,000,000different oligonucleotide probes. Such high density arrays comprise aprobe density of generally greater than about 60, more generally greaterthan about 100, most generally greater than about 600, often greatergreater than about 1000, more often greater than about 5,000, most oftengreater than about 10,000, preferably greater than about 40,000 morepreferably greater than about 100,000, and most preferably greater thanabout about 400,000 different oligonucleotide probes per cm². Theoligonucleotide probes range from about 5 to about 50 nucleotides, morepreferably from about 10 to about 40 nucleotides and most preferablyfrom about 15 to about 40 nucleotides in length. The array may comprisemore than 10, preferably more than 50, more preferably more than 100,and most preferably more than 1000 oligonucleotide probes specific foreach target gene. Although a planar array surface is preferred, thearray may be fabricated on a surface of virtually any shape or even amultiplicity of surfaces.

[0011] The array may further comprise mismatch control probes. Wheresuch mismatch controls are present, the quantifying step may comprisecalculating the difference in hybridization signal intensity betweeneach of the oligonucleotide probes and its corresponding mismatchcontrol probe. The quantifying may further comprise calculating theaverage difference in hybridization signal intensity between each of theoligonucleotide probes and its corresponding mismatch control probe foreach gene.

[0012] The probes present in the high density array can beoligonucleotide probes selected according to the optimization methodsdescribed below. Alternatively, non-optimal probes may be included inthe array, but the probes used for quantification (analysis) can beselected according to the optimization methods described below.

[0013] Oligonucleotide arrays for the practice of this invention arepreferably synthesized by light-directed very large scaled immobilizedpolymer synthesis (VLSIPS) as described herein. The array includes testprobes which are oligonucleotide probes each of which has a sequencethat is complementary to a subsequence of one of the genes (or the mRNAor the corresponding antisense cRNA) whose expression is to be detected.In addition, the array can contain normalization controls, mismatchcontrols and expression level controls as described herein.

[0014] The pool of nucleic acids may be labeled before, during, or afterhybridization, although in a preferred embodiment, the nucleic acids arelabeled before hybridization. Fluorescence labels are particularlypreferred and, where used, quantification of the hybridized nucleicacids is by quantification of fluorescence from the hybridizedfluorescently labeled nucleic acid. Such quantification is facilitatedby the use of a fluorescence microscope which can be equipped with anautomated stage to permit automatic scanning of the array, and which canbe equipped with a data acquisition system for the automated measurementrecording and subsequent processing of the fluorescence intensityinformation.

[0015] In a preferred embodiment, hybridization is at low stringency(e.g. about 20° C. to about 50° C., more preferably about 30° C. toabout 40° C., and most preferably about 37° C. and 6×SSPE-T or lower)with at least one wash at higher stringency. Hybridization may includesubsequent washes at progressively increasing stringency until a desiredlevel of hybridization specificity is reached.

[0016] The pool of target nucleic acids can be the total polyA⁺ mRNAisolated from a biological sample, or cDNA made by reverse transcriptionof the RNA or second strand cDNA or RNA transcribed from the doublestranded cDNA intermediate. Alternatively, the pool of target nucleicacids can be treated to reduce the complexity of the sample and therebyreduce the background signal obtained in hybridization. In one approach,a pool of mRNAs, derived from a biological sample, is hybridized with apool of oligonucleotides comprising the oligonucleotide probes presentin the high density array. The pool of hybridized nucleic acids is thentreated with RNase A which digests the single stranded regions. Theremaining double stranded hybridization complexes are then denatured andthe oligonucleotide probes are removed, leaving a pool of mRNAs enhancedfor those mRNAs complementary to the oligonucleotide probes in the highdensity array.

[0017] In another approach to background reduction, a pool of mRNAsderived from a biological sample is hybridized with paired targetspecific oligonucleotides where the paired target specificoligonucleotides are complementary to regions flanking subsequences ofthe mRNAs complementary to the oligonucleotide probes in the highdensity array. The pool of hybridized nucleic acids is treated withRNase H which digests the hybridized (double stranded) nucleic acidsequences. The remaining single stranded nucleic acid sequences whichhave a length about equivalent to the region flanked by the pairedtarget specific oligonucleotides are then isolated (e.g. byelectrophoresis) and used as the pool of nucleic acids for monitoringgene expression.

[0018] Finally, a third approach to background reduction involveseliminating or reducing the representation in the pool of particularpreselected target mRNA messages (e.g., messages that arecharacteristically overexpressed in the sample). This method involveshybridizing an oligonucleotide probe that is complementary to thepreselected target mRNA message to the pool of polyA⁺ mRNAs derived froma biological sample. The oligonucleotide probe hybridizes with theparticular preselected polyA⁺ mRNA (message) to which it iscomplementary. The pool of hybridized nucleic acids is treated withRNase H which digests the double stranded (hybridized) region therebyseparating the message from its polyA⁺ tail. Isolating or amplifying(e.g., using an oligo dT column) the polyA⁺ mRNA in the pool thenprovides a pool having a reduced or no representation of the preselectedtarget mRNA message.

[0019] It will be appreciated that the methods of this invention can beused to monitor (detect and/or quantify) the expression of any desiredgene of known sequence or subsequence. Moreover, these methods permitmonitoring expression of a large number of genes simultaneously andeffect significant advantages in reduced labor, cost and time. Thesimultaneous monitoring of the expression levels of a multiplicity ofgenes permits effective comparison of relative expression levels andidentification of biological conditions characterized by alterations ofrelative expression levels of various genes. Genes of particularinterest for expression monitoring include genes involved in thepathways associated with various pathological conditions (e.g., cancer)and whose expression is thus indicative of the pathological condition.Such genes include, but are not limited to the HER2 (c-erbB-2/neu)proto-oncogene in the case of breast cancer, receptor tyrosine kinases(RTKs) associated with the etiology of a number of tumors includingcarcinomas of the breast, liver, bladder, pancreas, as well asglioblastomas, sarcomas and squamous carcinomas, and tumor suppressorgenes such as the P53 gene and other “marker” genes such as RAS, MSH2,MLH1 and BRCA1. Other genes of particular interest for expressionmonitoring are genes involved in the immune response (e.g., interleukingenes), as well as genes involved in cell adhesion (e.g., the integrinsor selectins) and signal transduction (e.g., tyrosine kinases), etc.

[0020] In another embodiment, this invention provides for a method ofselecting a set of oligonucleotide probes, that specifically bind to atarget nucleic acid (e.g., a gene or genes whose expression is to bemonitored or nueleic acids derived from the gene or its transcribedmRNA). The method involves providing a high density array ofoligonucleotide probes where the array comprises a multiplicity ofprobes wherein each probe is complementary to a subsequence of thetarget nucleic acid. The target nucleic acid is then hybridized to thearray of oligonucleotide probes to identify and select those probeswhere the difference in hybridization signal intensity between eachprobe and its mismatch control is detectable (preferably greater thanabout 10% of the background signal intensity, more preferably greaterthan about 20% of the background signal intensity and most preferablygreater than about 50% of the background signal intensity). The methodcan further comprise hybridizing the array to a second pool of nucleicacids comprising nucleic acids other than the target nucleic acids; andidentifying and selecting probes having the lowest hybridization signaland where both the probe and its mismatch control have a hybridizationintensity equal to or less than about 5 times the background signalintensity, preferably equal to or less than about 2 times the backgroundsignal intensity, more preferably equal to or less than about 1 timesthe background signal intensity, and most preferably equal or less thanabout half the background signal intensity.

[0021] In a preferred embodiment, the multiplicity of probes can includeevery different probe of length n that is complementary to a subsequenceof the target nucleic acid. The probes can range from about 10 to about50 nucleotides in length. The array is preferably a high density arrayas described above. Similarly, the hybridization methods, conditions,times, fluid volumes, detection methods are as described above andherein below.

[0022] In addition, this invention provides for a composition comprisingan array of oligonucleotide probes immobilized on a substrate, where thearray comprises more than 100 different oligonucleotides and eachdifferent oligonucleotide is localized in a predetermined region of thesolid support and the density of the array is greater than about 60different oligonucleotides per 1 cm² of substrate. The oligonucleotideprobes are specifically hybridized to one or more fluorescently labelednucleic acids such that the fluorescence in each region of the array isindicative of the level of expression of each of a multiplicity ofpreselected genes. The array is preferably a high density array asdescribed above and may further comprise expression level controls,mismatch controls and normalization controls as described herein.

[0023] Finally, this invention provides for kits for simultaneouslymonitoring expression levels of a multiplicity of genes. The kitsinclude an array of immobilized oligonucleotide probes complementary tosubsequences of the multiplicity of target genes, as described above. Inone embodiment, the array comprises at least 100 differentoligonucleotide probes and the density of the array is greater thanabout 60 different oligonucleotides per 1 cm² of surface. The kit mayalso include instructions describing the use of the array for detectionand/or quantification of expression levels of the multiplicity of genes.The kit may additionally include one or more of the following: buffers,hybridization mix, wash and read solutions, labels, labeling reagents(enzymes etc.), “control” nucleic acids, software for probe selection,array reading or data analysis and any of the other materials orreagents described herein for the practice of the claimed methods.

[0024] Definitions.

[0025] The phrase “massively parallel screening” refers to thesimultaneous screening of at least about 100, preferably about 1000,more preferably about 10,000 and most preferably about 1,000,000different nucleic acid hybridizations.

[0026] The terms “nucleic acid” or “nucleic acid molecule” refer to adeoxyribonucleotide or ribonucleotide polymer in either single-ordouble-stranded form, and unless otherwise limited, would encompassknown analogs of natural nucleotides that can function in a similarmanner as naturally occurring nucleotides.

[0027] An oligonucleotide is a single-stranded nucleic acid ranging inlength from 2 to about 500 bases.

[0028] As used herein a “probe” is defined as an oligonucleotide capableof binding to a target nucleic acid of complementary sequence throughone or more types of chemical bonds, usually through complementary basepairing, usually through hydrogen bond formation. As used herein, anoligonucleotide probe may include natural (ie. A, G, C, or T) ormodified bases (7-deazaguanosine, inosine, etc.). In addition, the basesin oligonucleotide probe may be joined by a linkage other than aphosphodiester bond, so long as it does not interfere withhybridization. Thus, oligonucleotide probes may be peptide nucleic acidsin which the constituent bases are joined by peptide bonds rather thanphosphodiester linkages.

[0029] The term “target nucleic acid” refers to a nucleic acid (oftenderived from a biological sample), to which the oligonucleotide probe isdesigned to specifically hybridize. It is either the presence or absenceof the target nucleic acid that is to be detected, or the amount of thetarget nucleic acid that is to be quantified. The target nucleic acidhas a sequence that is complementary to the nucleic acid sequence of thecorresponding probe directed to the target. The term target nucleic acidmay refer to the specific subsequence of a larger nucleic acid to whichthe probe is directed or to the overall sequence (e.g., gene or mRNA)whose expression level it is desired to detect. The difference in usagewill be apparent from context.

[0030] “Subsequence” refers to a sequence of nucleic acids that comprisea part of a longer sequence of nucleic acids.

[0031] The term “complexity” is used here according to standard meaningof this term as established by Britten et al. Methods of Enzymol. 29:363(1974). See, also Cantor and Schimmel Biophysical Chemistry: Pan III at1228-1230 for further explanation of nucleic acid complexity.

[0032] “Bind(s) substantially” refers to complementary hybridizationbetween a probe nucleic acid and a target nucleic acid and embracesminor mismatches that can be accommodated by reducing the stringency ofthe hybridization media to achieve the desired detection of the targetpolynucleotide sequence.

[0033] The phrase “hybridizing specifically to”, refers to the binding,duplexing, or hybridizing of a molecule only to a particular nucleotidesequence under stringent conditions when that sequence is present in acomplex mixture (e.g., total cellular) DNA or RNA. The term “stringentconditions” refers to conditions under which a probe will hybridize toits target subsequence, but to no other sequences. Stringent conditionsare sequence-dependent and will be different in different circumstances.Longer sequences hybridize specifically at higher temperatures.Generally, stringent conditions are selected to be about 5° C. lowerthan the thermal melting point (Tm) for the specific sequence at adefined ionic strength and pH. The Tm is the temperature (under definedionic strength, pH, and nucleic acid concentration) at which 50% of theprobes complementary to the target sequence hybridize to the targetsequence at equilibrium. (As the target sequences are generally presentin excess, at Tm, 50% of the probes are occupied at equilibrium).Typically, stringent conditions will be those in which the saltconcentration is at least about 0.01 to 1.0 M Na ion concentration (orother salts) at pH 7.0 to 8.3 and the temperature is at least about 30°C. for short probes (e.g., 10 to 50 nucleotides). Stringent conditionsmay also be achieved with the addition of destabilizing agents such asformamide.

[0034] The term “mismatch control” refers to a probe that has a sequencedeliberately selected not to be perfectly complementary to a particulartarget sequence. The mismatch control typically has a corresponding testprobe that is perfectly complementary to the same particular targetsequence. The mismatch may comprise one or more bases. While themismatch(s) may be locates anywhere in the mismatch probe, terminalmismatches are less desirable as a terminal mismatch is less likely toprevent hybridization of the target sequence. In a particularlypreferred embodiment, the mismatch is located at or near the center ofthe probe such that the mismatch is most likely to destabilize theduplex with the target sequence under the test hybridization conditions.

[0035] The terms “background” or “background signal intensity” refer tohybridization signals resulting from non-specific binding, or otherinteractions, between the labeled target nucleic acids and components ofthe oligonucleotide array (e.g., the oligonucleotide probes, controlprobes, the array substrate, etc.). Background signals may also beproduced by intrinsic fluorescence of the array components themselves. Asingle background signal can be calculated for the entire array, or adifferent background signal may be calculated for each target nucleicacid. In a preferred embodiment, background is calculated as the averagehybridization signal intensity for the lowest 5% to 10% of the probes inthe array, or, where a different background signal is calculated foreach target gene, for the lowest 5% to 10% of the probes for each gene.Of course, one of skill in the art will appreciate that where the probesto a particular gene hybridize well and thus appear to be specificallybinding to a target sequence, they should not be used in a backgroundsignal calculation. Alternatively, background may be calculated as theaverage hybridization signal intensity produced by hybridization toprobes that are not complementary to any sequence found in the sample(e.g. probes directed to nucleic acids of the opposite sense or to genesnot found in the sample such as bacterial genes where the sample ismammalian nucleic acids). Background can also be calculated as theaverage signal intensity produced by regions of the array that lack anyprobes at all.

[0036] The term “quantifying” when used in the context of quantifyingtranscription levels of a gene can refer to absolute or to relativequantification. Absolute quantification may be accomplished by inclusionof known concentration(s) of one or more target nucleic acids (e.g.control nucleic acids such as Bio B or with known amounts the targetnucleic acids themselves) and referencing the hybridization intensity ofunknowns with the known target nucleic acids (e.g. through generation ofa standard curve). Alternatively, relative quantification can beaccomplished by comparison of hybridization signals between two or moregenes, or between two or more treatments to quantify the changes inhybridization intensity and, by implication, transcription level.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 shows a plot of hybridization intensity plotted as afunction of concentration of target mRNA. Graphs A and B show thehybridization intensity of IL-4 RNA hybridized to the high density arrayof Example 1. Graph B expands the ordinate of graph A to show the lowconcentration values. Graphs C and D show hybridization intensityplotted as a function of target RNA for a collection of different targetRNAs. The graphs show the average values of the 1000 highest intensityprobes. Graph D expands the ordinate of graph C to show the lowconcentration values.

[0038]FIG. 2 shows a plot of hybridization intensity for mouse libraryRNA, mouse library RNA spiked with mCTLA8, IL-6, IL-3, IFN-γ, andIL-12p40 at 10 pM or 50 pM. The data presented is based uponapproximately the best (optimal) 10% of the probes to each gene, wherethe optimal probes are selected according to the method disclosedherein.

[0039]FIG. 3 shows a plot of the data from Example 1 (FIG. 2) with theordinate condensed to show the constitutively expressed GAPDH and Actingenes and the intrinsic expressed IL-10 gene.

DETAILED DESCRIPTION

[0040] This invention provides methods of monitoring (detecting and/orquantifying) the expression levels of one or more genes. The methodsinvolve hybridization of a nucleic acid target sample to a high densityarray of nucleic acid probes and then quantifying the amount of targetnucleic acids hybridized to each probe in the array.

[0041] While nucleic acid hybridization has been used for some time todetermine the expression levels of various genes (e.g., Northern Blot),it was a surprising discovery of this invention that high density arraysare suitable for the quantification of the small variations inexpression (transcription) levels of a gene in the presence of a largepopulation of heterogenous nucleic acids. The signal may be present at aconcentration of less than about 1 in 1,000, and is often present at aconcentration less than 1 in 10,000 more preferably less than about 1 in50,000 and most preferably less than about 1 in 100,000 or even 1 in1,000,000.

[0042] Prior to this invention, it was expected that hybridization ofsuch a complex mixture to a high density array might overwhelm theavailable probes and make it impossible to detect the presence oflow-level target nucleic acids. It was thus unclear that a low levelsignal could be isolated and detected in the presence of misleadingsignals due to cross-hybridization and non-specific binding both tosubstrate and probe.

[0043] It was a surprising discovery that, to the contrary, high densityarrays are particularly well suited for monitoring expression of amultiplicity of genes and provide a level of sensitivity anddiscrimination hitherto unexpected.

[0044] Preferred high density arrays of this invention comprise greaterthan about 100, preferably greater than about 1000, more preferablygreater than about 16,000 and most preferably greater than about 65,000or 250,000 or even greater than about 1,000,000 differentoligonucleotide probes. The oligonucleotide probes range from about 5 toabout 50 nucleotides, more preferably from about 10 to about 40nucleotides and most preferably from about 15 to about 40 nucleotides inlength.

[0045] The location and sequence of each different oligonucleotide probesequence in the array is known. Moreover, the large number of differentprobes occupies a relatively small area providing a high density arrayhaving a probe density of generally greater than about 60, moregenerally greater than about 100, most generally greater than about 600,often greater greater than about 1000, more often greater than about5,000, most often greater than about 10,000, preferably greater thanabout 40,000 more preferably greater than about 100,000, and mostpreferably greater than about about 400,000 different oligonucleotideprobes per cm². The small surface area of the array (often less thanabout 10 cm², preferably less than about 5 cm² more preferably less thanabout 2 cm², and most preferably less than about 1.6 cm_(—) ²) permitsextremely uniform hybridization conditions (temperature regulation, saltcontent, etc.) while the extremely large number of probes allowsmassively parallel processing of hybridizations.

[0046] It was a discovery of this invention that the use of high densityarrays for expression monitoring provides a number of advantages notfound with other methods. For example, the use of large numbers ofdifferent probes that specifically bind to the transcription product ofa particular target gene provides a high degree of redundancy andinternal control that permits optimization of probe sets for effectivedetection of particular target genes and minimizes the possibility oferrors due to cross-reactivity with other nucleic acid species.

[0047] Apparently suitable probes often prove ineffective for expressionmonitoring by hybridization. For example, certain subsequences of aparticular target gene may be found in other regions of the genome andprobes directed to these subsequences will cross-hybridize with theother regions and not provide a signal that is a meaningful measure ofthe expression level of the target gene. Even probes that show littlecross reactivity may be unsuitable because they generally show poorhybridization due to the formation of structures that prevent effectivehybridization. Finally, in sets with large numbers of probes, it isdifficult to identify hybridization conditions that are optimal for allthe probes in a set. Because of the high degree of redundancy providedby the large number of probes for each target gene, it is possible toeliminate those probes that function poorly under a given set ofhybridization conditions and still retain enough probes to a particulartarget gene to provide an extremely sensitive and reliable measure ofthe expression level (transcription level) of that gene.

[0048] In addition, the use of large numbers of different probes to eachtarget gene makes it possible to monitor expression of families ofclosely-related nucleic acids. The probes may be selected to hybridizeboth with subsequences that are conserved across the family and withsubsequences that differ in the different nucleic acids in the family.Thus, hybridization with such arrays permits simultaneous monitoring ofthe various members of a gene family even where the various genes areapproximately the same size and have high levels of homology. Suchmeasurements are difficult or impossible with traditional hybridizationmethods.

[0049] Because the high density arrays contain such a large number ofprobes it is possible to provide numerous controls including, forexample, controls for variations or mutations in a particular gene,controls for overall hybridization conditions, controls for samplepreparation conditions, controls for metabolic activity of the cell fromwhich the nucleic acids are derived and mismatch controls fornon-specific binding or cross hybridization.

[0050] Finally, because of the small area occupied by the high densityarrays, hybridization may be carried out in extremely small fluidvolumes (e.g., 250 μl or less, more preferably 100 μl or less, and mostpreferably 10 μl or less). In small volumes, hybridization may proceedvery rapidly. In addition, hybridization conditions are extremelyuniform throughout the sample, and the hybridization format is amenableto automated processing.

[0051] This invention demonstrates that hybridization with high densityoligonucleotide probe arrays provides an effective means of monitoringexpression of a multiplicity of genes. In addition this inventionprovides for methods of sample treatment and array designs and methodsof probe selection that optimize signal detection at extremely lowconcentrations in complex nucleic acid mixtures.

[0052] The expression monitoring methods of this invention may be usedin a wide variety of circumstances including detection of disease,identification of differential gene expression between two samples(e.g., a pathological as compared to a healthy sample), screening forcompositions that upregulate or downregulate the expression ofparticular genes, and so forth.

[0053] In one preferred embodiment, the methods of this invention areused to monitor the expression (transcription) levels of nucleic acidswhose expression is altered in a disease state. For example, a cancermay be characterized by the overexpression of a particular marker suchas the HER2 (c-erbB-2/neu) proto-oncogene in the case of breast cancer.Similarly, overexpression of receptor tyrosine kinases (RTKs) isassociated with the etiology of a number of tumors including carcinomasof the breast, liver, bladder, pancreas, as well as glioblastomas,sarcomas and squamous carcinomas (see Carpenter, Ann. Rev. Biochem., 56:881-914 (1987)). Conversely, a cancer (e.g., colerectal, lung andbreast) may be characterized by the mutation of or underexpression of atumor suppressor gene such as P53 (see, e.g., Tominaga et al. CriticalRev. in Oncogenesis, 3: 257-282 (1992)).

[0054] The materials and methods of this invention are typically used tomonitor the expression of a multiplicity of different genessimultaneously. Thus, in one embodiment, the invention provide forsimultaneous monitoring of at least about 10, preferably at least about100, more preferably at least about 1000 and most preferably at leastabout 10,000 different genes.

[0055] I. Methods of Monitoring Gene Expression.

[0056] Generally the methods of monitoring gene expression of thisinvention involve (1) providing a pool of target nucleic acidscomprising RNA transcript(s) of one or more target gene(s), or nucleicacids derived from the RNA transcript(s); (2) hybridizing the nucleicacid sample to a high density array of probes (including controlprobes); and (3) detecting the hybridized nucleic acids and calculatinga relative expression (transcription) level.

[0057] A) Providing a Nucleic Acid Sample.

[0058] One of skill in the art will appreciate that in order to measurethe transcription level (and thereby the expression level) of a gene orgenes, it is desirable to provide a nucleic acid sample comprising mRNAtranscript(s) of the gene or genes, or nucleic acids derived from themRNA transcript(s). As used herein, a nucleic acid derived from an mRNAtranscript refers to a nucleic acid for whose synthesis the mRNAtranscript or a subsequence thereof has ultimately served as a template.Thus, a cDNA reverse transcribed from an mRNA, an RNA transcribed fromthat cDNA, a DNA amplified from the cDNA, an RNA transcribed from theamplified DNA, etc., are all derived from the mRNA transcript anddetection of such derived products is indicative of the presence and/orabundance of the original transcript in a sample. Thus, suitable samplesinclude, but are not limited to, mRNA transcripts of the gene or genes,cDNA reverse transcribed from the mRNA, cRNA transcribed from the cDNA,DNA amplified from the genes, RNA transcribed from amplified DNA, andthe like.

[0059] In a particularly preferred embodiment, where it is desired toquantify the transcription level (and thereby expression) of a one ormore genes in a sample, the nucleic acid sample is one in which theconcentration of the mRNA transcript(s) of the gene or genes, or theconcentration of the nucleic acids derived from the mRNA transcript(s),is proportional to the transcription level (and therefore expressionlevel) of that gene. Similarly, it is preferred that the hybridizationsignal intensity be proportional to the amount of hybridized nucleicacid. While it is preferred that the proportionality be relativelystrict (e.g., a doubling in transcription rate results in a doubling inmRNA transcript in the sample nucleic acid pool and a doubling inhybridization signal), one of skill will appreciate that theproportionality can be more relaxed and even non-linear. Thus, forexample, an assay where a 5 fold difference in concentration of thetarget mRNA results in a 3 to 6 fold difference in hybridizationintensity is sufficient for most purposes. Where more precisequantification is required appropriate controls can be run to correctfor variations introduced in sample preparation and hybridization asdescribed herein. In addition, serial dilutions of “standard” targetmRNAs can be used to prepare calibration curves according to methodswell known to those of skill in the art. Of course, where simpledetection of the presence or absence of a transcript is desired, noelaborate control or calibration is required.

[0060] In the simplest embodiment, such a nucleic acid sample is thetotal mRNA isolated from a biological sample. The term “biologicalsample”, as used herein, refers to a sample obtained from an organism orfrom components (e.g., cells) of an organism. The sample may be of anybiological tissue or fluid. Frequently the sample will be a “clinicalsample” which is a sample derived from a patient. Such samples include,but are not limited to, sputum, blood, blood cells (e.g., white cells),tissue or fine needle biopsy samples, urine, peritoneal fluid, andpleural fluid, or cells therefrom. Biological samples may also includesections of tissues such as frozen sections taken for histologicalpurposes.

[0061] The nucleic acid (either genomic DNA or mRNA) may be isolatedfrom the sample according to any of a number of methods well known tothose of skill in the art. One of skill will appreciate that wherealterations in the copy number of a gene are to be detected genomic DNAis preferably isolated. Conversely, where expression levels of a gene orgenes are to be detected, preferably RNA (mRNA) is isolated.

[0062] Methods of isolating total mRNA are well known to those of skillin the art. For example, methods of isolation and purification ofnucleic acids are described in detail in Chapter 3 of LaboratoryTechniques in Biochemistry and Molecular Biology: Hybridization WithNucleic Acid Probes, Part I. Theory and Nucleic Acid Preparation, P.Tijssen, ed. Elsevier, N.Y. (1993) and Chapter 3 of LaboratoryTechniques in Biochemistry and Molecular Biology: Hybridization WithNucleic Acid Probes, Part I. Theory and Nucleic Acid Preparation, P.Tijssen, ed. Elsevier, N.Y. (1993)).

[0063] In a preferred embodiment, the total nucleic acid is isolatedfrom a given sample using, for example, an acidguanidinium-phenol-chloroform extraction method and polyA⁺ mRNA isisolated by oligo dT column chromatography or by using (dT)n magneticbeads (see, e.g., Sambrook et al., Molecular Cloning: A LaboratoryManual (2nd ed.), Vols. 1-3, Cold Spring Harbor Laboratory, (1989), orCurrent Protocols in Molecular Biology, F. Ausubel et al., ed. GreenePublishing and Wiley-Interscience, New York (1987)).

[0064] Frequently, it is desirable to amplify the nucleic acid sampleprior to hybridization. One of skill in the art will appreciate thatwhatever amplification method is used, if a quantitative result isdesired, care must be taken to use a method that maintains or controlsfor the relative frequencies of the amplified nucleic acids.

[0065] Methods of “quantitative” amplification are well known to thoseof skill in the art. For example, quantitative PCR involvessimultaneously co-amplifying a known quantity of a control sequenceusing the same primers. This provides an internal standard that may beused to calibrate the PCR reaction. The high density array may theninclude probes specific to the internal standard for quantification ofthe amplified nucleic acid.

[0066] One preferred internal standard is a synthetic AW106 cRNA. TheAW106 cRNA is combined with RNA isolated from the sample according tostandard techniques known to those of skill in the art. The RNA is thenreverse transcribed using a reverse transcriptase to provide copy DNA.The cDNA sequences are then amplified (e.g., by PCR) using labeledprimers. The amplification products are separated, typically byelectrophoresis, and the amount of radioactivity (proportional to theamount of amplified product) is determined. The amount of mRNA in thesample is then calculated by comparison with the signal produced by theknown AW106 RNA standard. Detailed protocols for quantitative PCR areprovided in PCR Protocols, A Guide to Methods and Applications, Innis etal., Academic Press, Inc. N.Y., (1990).

[0067] Other suitable amplification methods include, but are not limitedto polymerase chain reaction (PCR) (Innis, et al., PCR Protocols. Aguide to Methods and Application. Academic Press, Inc. San Diego,(1990)), ligase chain reaction (LCR) (see Wu and Wallace, Genomics, 4:560 (1989), Landegren, et al., Science, 241: 1077 (1988) and Barringer,et al., Gene, 89: 117 (1990), transcription amplification (Kwoh, et al.,Proc. Natl. Acad. Sci. USA, 86: 1173 (1989)), and self-sustainedsequence replication (Guatelli, et al., Proc. Nat. Acad. Sci. USA, 87:1874 (1990)).

[0068] In a particularly preferred embodiment, the sample mRNA isreverse transcribed with a reverse transcriptase and a promer consistingof oligo dT and a sequence encoding the phage T7 promoter to providesingle stranded DNA template. The second DNA strand is polymerized usinga DNA polymerase. After synthesis of double-stranded cDNA, T7 RNApolymerase is added and RNA is transcribed from the cDNA template.Successive rounds of transcription from each single cDNA templateresults in amplified RNA. Methods of in vitro polymerization are wellknown to those of skill in the art (see, e.g., Sambrook, supra.) andthis particular method is described in detail by Van Gelder, et al.,Proc. Natl. Acad. Sci. USA, 87: 1663-1667 (1990) who demonstrate that invitro amplification according to this method preserves the relativefrequencies of the various RNA transcripts. Moreover, Eberwine et al.Proc. Natl. Acad. Sci. USA, 89: 3010-3014 provide a protocol that usestwo rounds of amplification via in vitro transcription to achievegreater than 10⁶ fold amplification of the original starting materialthereby permiting expression monitoring even where biological samplesare limited.

[0069] It will be appreciated by one of skill in the art that the directtranscription method described above provides an antisense (aRNA) pool.Where antisense RNA is used as the target nucleic acid, theoligonucleotide probes provided in the array are chosen to becomplementary to subsequences of the antisense nucleic acids.Conversely, where the target nucleic acid pool is a pool of sensenucleic acids, the oligonucleotide probes are selected to becomplementary to subsequences of the sense nucleic acids. Finally, wherethe nucleic acid pool is double stranded, the probes may be of eithersense as the target nucleic acids include both sense and antisensestrands.

[0070] The protocols cited above include methods of generating pools ofeither sense or antisense nucleic acids. Indeed, one approach can beused to generate either sense or antisense nucleic acids as desired. Forexample, the cDNA can be directionally cloned into a vector (e.g.,Stratagene's p Bluscript II KS (+) phagemid) such that it is flanked bythe T3 and 7 promoters. In vitro transcription with the T3 polymerasewill produce RNA of one sense (the sense depending on the orientation ofthe insert), while in vitro transcription with the T7 polymerase willproduce RNA having the opposite sense. Other suitable cloning systemsinclude phage lamda vectors designed for Cre-loxP plasmid subcloning(see e.g., Palazzolo et al., Gene, 88: 25-36 (1990)).

[0071] In a particularly preferred embodiment, a high activity RNApolymerase (e.g. about 2500 units/μL for T7, available from EpicentreTechnologies) is used.

[0072] B) Labeling Nucleic Acids.

[0073] In a preferred embodiment, the hybridized nucleic acids aredetected by detecting one or more labels attached to the sample nucleicacids. The labels may be incorporated by any of a number of means wellknown to those of skill in the art. However, in a preferred embodiment,the label is simultaneously incorporated during the amplification stepin the preparation of the sample nucleic acids. Thus, for example,polymerase chain reaction (PCR) with labeled primers or labelednucleotides will provide a labeled amplification product. In a preferredembodiment, transcription amplification, as described above, using alabeled nucleotide (e.g. fluorescein-labeled UTIP and/or CTP)incorporates a label into the transcribed nucleic acids.

[0074] Alternatively, a label may be added directly to the originalnucleic acid sample (e.g., mRNA, polyA mRNA, cDNA, etc.) or to theamplification product after the amplification is completed. Means ofattaching labels to nucleic acids are well known to those of skill inthe art and include, for example nick translation or end-labeling (e.g.with a labeled RNA) by kinasing of the nucleic acid and subsequentattachment (ligation) of a nucleic acid linker joining the samplenucleic acid to a label (e.g., a fluorophore).

[0075] Detectable labels suitable for use in the present inventioninclude any composition detectable by spectroscopic, photochemical,biochemical, immunochemical, electrical, optical or chemical means.Useful labels in the present invention include biotin for staining withlabeled streptavidin conjugate, magnetic beads (e.g., Dynabeads™),fluorescent dyes (e.g., fluorescein, texas red, rhodamine, greenfluorescent protein, and the like), radiolabels (e.g., ³H, ¹²⁵I, ³⁵S,¹⁴C, or ³²P), enzymes (e.g., horse radish peroxidase, alkalinephosphatase and others commonly used in an ELISA), and colorimetriclabels such as colloidal gold or colored glass or plastic (e.g.,polystyrene, polypropylene, latex, etc.) beads. Patents teaching the useof such labels include U.S. Pat. Nos. 3,817,837; 3,850,752; 3,939,350;3,996,345; 4,277,437; 4,275,149; and 4,366,241.

[0076] Means of detecting such labels are well known to those of skillin the art. Thus, for example, radiolabels may be detected usingphotographic film or scintillation counters, fluorescent markers may bedetected using a photodetector to detect emitted light. Enzymatic labelsare typically detected by providing the enzyme with a substrate anddetecting the reaction product produced by the action of the enzyme onthe substrate, and colorimetric labels are detected by simplyvisualizing the colored label.

[0077] The label may be added to the target (sample) nucleic acid(s)prior to, or after the hybridization. So called “direct labels” aredetectable labels that are directly attached to or incorporated into thetarget (sample) nucleic acid prior to hybridization. In contrast, socalled “indirect labels” are joined to the hybrid duplex afterhybridization. Often, the indirect label is attached to a binding moietythat has been attached to the target nucleic acid prior to thehybridization. Thus, for example, the target nucleic acid may bebiotinylated before the hybridization. After hybridization, anaviden-conjugated fluorophore will bind the biotin bearing hybridduplexes providing a label that is easily detected. For a detailedreview of methods of labeling nucleic acids and detecting labeledhybridized nucleic acids see Laboratory Techniques in Biochemistry andMolecular Biology, Vol. 24: Hybridization With Nucleic Acid Probes, P.Tijssen, ed. Elsevier, N.Y., (1993)).

[0078] Fluorescent labels are preferred and easily added during an invitro transcription reaction. In a preferred embodiment, fluoresceinlabeled UTP and CTP are incorporated into the RNA produced in an invitro transcription reaction as described above.

[0079] C) Modifying Sample to Improve Signal/Noise Ratio.

[0080] The nucleic acid sample may be modified prior to hybridization tothe high density probe array in order to reduce sample complexitythereby decreasing background signal and improving sensitivity of themeasurement. In one embodiment, complexity reduction is achieved byselective degradation of background mRNA. This is accomplished byhybridizing the sample mRNA (e.g., polyA⁺ RNA) with a pool of DNAoligonucleotides that hybridize specifically with the regions to whichthe probes in the array specifically hybridize. In a preferredembodiment, the pool of oligonucleotides consists of the same probeoligonucleotides as found on the high density array.

[0081] The pool of oligonucleotides hybridizes to the sample mRNAforming a number of double stranded (hybrid duplex) nucleic acids. Thehybridized sample is then treated with RNase A, a nuclease thatspecifically digests single stranded RNA. The RNase A is then inhibited,using a protease and/or commercially available RNase inhibitors, and thedouble stranded nucleic acids are then separated from the digestedsingle stranded RNA. This separation may be accomplished in a number ofways well known to those of skill in the art including, but not limitedto, electrophoresis, and gradient centrifugation. However, in apreferred embodiment, the pool of DNA oligonucleotides is providedattached to beads forming thereby a nucleic acid affinity column. Afterdigestion with the RNase A, the hybridized DNA is removed simply bydenaturing (e.g., by adding heat or increasing salt) the hybrid duplexesand washing the previously hybridized mRNA off in an elution buffer.

[0082] The undigested mRNA fragments which will be hybridized to theprobes in the high density array are then preferably end-labeled with afluorophore attached to an RNA linker using an RNA ligase. Thisprocedure produces a labeled sample RNA pool in which the nucleic acidsthat do not correspond to probes in the array are eliminated and thusunavailable to contribute to a background signal.

[0083] Another method of reducing sample complexity involves hybridizingthe mRNA with deoxyoligonucleotides that hybridize to regions thatborder on either size the regions to which the high density array probesare directed. Treatment with RNAse H selectively digests the doublestranded (hybrid duplexes) leaving a pool of single-stranded mRNAcorresponding to the short regions (e.g., 20 mer) that were formerlybounded by the deoxyolignucleotide probes and which correspond to thetargets of the high density array probes and longer mRNA sequences thatcorrespond to regions between the targets of the probes of the highdensity array. The short RNA fragments are then separated from the longfragments (e.g., by electrophoresis), labeled if necessary as describedabove, and then are ready for hybridization with the high density probearray.

[0084] In a third approach, sample complexity reduction involves theselective removal of particular (preselected) mRNA messages. Inparticular, highly expressed mRNA messages that are not specificallyprobed by the probes in the high density array are preferably removed.This approach involves hybridizing the polyA⁺ mRNA with anoligonucleotide probe that specifically hybridizes to the preselectedmessage close to the 3′ (poly A) end. The probe may be selected toprovide high specificity and low cross reactivity. Treatment of thehybridized message/probe complex with RNase H digests the doublestranded region effectively removing the polyA⁺ tail from the rest ofthe message. The sample is then treated with methods that specificallyretain or amplify polyA⁺ RNA (e.g., an oligo dT column or (dT)n magneticbeads). Such methods will not retain or amplify the selected message(s)as they are no longer associated with a polyA⁺ tail. These highlyexpressed messages are effectively removed from the sample providing asample that has reduced background mRNA.

[0085] II. Hybridization Array Design.

[0086] A) Probe Composition.

[0087] One of skill in the art will appreciate that an enormous numberof array designs are suitable for the practice of this invention. Thehigh density array will typically include a number of probes thatspecifically hybridize to the nucleic acid expression of which is to bedetected. In addition, in a preferred embodiment, the array will includeone or more control probes.

[0088] 1) Test Probes.

[0089] In its simplest embodiment, the high density array includes “testprobes”. These are oligonucleotides that range from about 5 to about 50nucleotides, more preferably from about 10 to about 40 nucleotides andmost preferably from about 15 to about 40 nucleotides in length. Theseoligonucleotide probes have sequences complementary to particularsubsequences of the genes whose expression they are designed to detect.Thus, the test probes are capable of specifically hybridizing to thetarget nucleic acid they are to detect.

[0090] In addition to test probes that bind the target nucleic acid(s)of interest, the high density array can contain a number of controlprobes. The control probes fall into three categories referred to hereinas 1) Normalization controls; 2) Expression level controls; and 3)Mismatch controls.

[0091] 2) Normalization Control

[0092] Normalization controls are oligonucleotide probes that areperfectly complementary to labeled reference oligonucleotides that areadded to the nucleic acid sample. The signals obtained from thenormalization controls after hybridization provide a control forvariations in hybridization conditions, label intensity, “reading”efficiency and other factors that may cause the signal of a perfecthybridization to vary between arrays. In a preferred embodiment, signals(e.g., fluorescence intensity) read from all other probes in the arrayare divided by the signal (e.g., fluorescence intensity) from thecontrol probes thereby normalizing the measurements.

[0093] Virtually any probe may serve as a normalization control.However, it is recognized that hybridization efficiency varies with basecomposition and probe length. Preferred normalization probes areselected to reflect the average length of the other probes present inthe array, however, they can be selected to cover a range of lengths.The normalization control(s) can also be selected to reflect the(average) base composition of the other probes in the array, however ina preferred embodiment, only one or a few normalization probes are usedand they are selected such that they hybridize well (i.e. no secondarystructure) and do not match any target-specific probes.

[0094] Normalization probes can be localized at any position in thearray or at multiple positions throughout the array to control forspatial variation in hybridization efficiently. In a preferredembodiment, the normalization controls are located at the corners oredges of the array as well as in the middle.

[0095] 3) Expression Level Controls.

[0096] Expression level controls are probes that hybridize specificallywith constitutively expressed genes in the biological sample. Expressionlevel controls are designed to control for the overall health andmetabolic activity of a cell. Examination of the covariance of anexpression level control with the expression level of the target nucleicacid indicates whether measured changes or variations in expressionlevel of a gene is due to changes in transcription rate of that gene orto general variations in health of the cell. Thus, for example, when acell is in poor health or lacking a critical metabolite the expressionlevels of both an active target gene and a constitutively expressed geneare expected to decrease. The converse is also true. Thus where theexpression levels of both an expression level control and the targetgene appear to both decrease or to both increase, the change may beattributed to changes in the metabolic activity of the cell as a whole,not to differential expression of the target gene in question.Conversely, where the expression levels of the target gene and theexpression level control do not covary, the variation in the expressionlevel of the target gene is attributed to differences in regulation ofthat gene and not to overall variations in the metabolic activity of thecell.

[0097] Virtually any constitutively expressed gene provides a suitabletarget for expression level controls. Typically expression level controlprobes have sequences complementary to subsequences of constitutivelyexpressed “housekeeping genes” including, but not limited to the β-actingene, the transferrin receptor gene, the GAPDH gene, and the like.

[0098] 4) Mismatch Controls.

[0099] Mismatch controls may also be provided for the probes to thetarget genes, for expression level controls or for normalizationcontrols. Mismatch controls are oligonucleotide probes identical totheir corresponding test or control probes except for the presence ofone or more mismatched bases. A mismatched base is a base selected sothat it is not complementary to the corresponding base in the targetsequence to which the probe would otherwise specifically hybridize. Oneor more mismatches are selected such that under appropriatehybridization conditions (e.g. stringent conditions) the test or controlprobe would be expected to hybridize with its target sequence, but themismatch probe would not hybridize (or would hybridize to asignificantly lesser extent). Preferred mismatch probes contain acentral mismatch. Thus, for example, where a probe is a 20 mer, acorresponding mismatch probe will have the identical sequence except fora single base mismatch (e.g., substituting a G, a C or a T for an A) atany of positions 6 through 14 (the central mismatch).

[0100] Mismatch probes thus provide a control for non-specific bindingor cross-hybridization to a nucleic acid in the sample other than thetarget to which the probe is directed. Mismatch probes thus indicatewhether a hybridization is specific or not. For example, if the targetis present the perfect match probes should be consistently brighter thanthe mismatch probes. In addition, if all central mismatches are present,the mismatch probes can be used to detect a mutation. Finally, it wasalso a discovery of the present invention that the difference inintensity between the perfect match and the mismatch probe (I(PM)-I(MM))provides a good measure of the concentration of the hybridized material.

[0101] 5) Sample Preparation/Amplification Controls.

[0102] The high density array may also include samplepreparation/amplification control probes. These are probes that arecomplementary to subsequences of control genes selected because they donot normally occur in the nucleic acids of the particular biologicalsample being assayed. Suitable sample preparation/amplification controlprobes include, for example, probes to bacterial genes (e.g., Bio B)where the sample in question is a biological from a eukaryote.

[0103] The RNA sample is then spiked with a known amount of the nucleicacid to which the sample preparation/amplification control probe isdirected before processing. Quantification of the hybridization of thesample preparation/amplification control probe then provides a measureof alteration in the abundance of the nucleic acids caused by processingsteps (e.g. PCR, reverse transcription, in vitro transcription, etc.).

[0104] B) “Test Probe” Selection and Optimization.

[0105] In a preferred embodiment, oligonucleotide probes in the highdensity array are selected to bind specifically to the nucleic acidtarget to which they are directed with minimal non-specific binding orcross-hybridization under the particular hybridization conditionsutilized. Because the high density arrays of this invention can containin excess of 1,000,000 different probes, it is possible to provide everyprobe of a characteristic length that binds to a particular nucleic acidsequence. Thus, for example, the high density array can contain everypossible 20 mer sequence complementary to an IL-2 mRNA.

[0106] There, however, may exist 20 mer subsequences that are not uniqueto the IL-2 mRNA. Probes directed to these subsequences are expected tocross hybridize with occurrences of their complementary sequence inother regions of the sample genome. Similarly, other probes simply maynot hybridize effectively under the hybridization conditions (e.g., dueto secondary structure, or interactions with the substrate or otherprobes). Thus, in a preferred embodiment, the probes that show such poorspecificity or hybridization efficiency are identified and may not beincluded either in the high density array itself (e.g., duringfabrication of the array) or in the post-hybridization data analysis.

[0107] Thus, in one embodiment, this invention provides for a method ofoptimizing a probe set for detection of a particular gene. Generally,this method involves providing a high density array containing amultiplicity of probes of one or more particular length(s) that arecomplementary to subsequences of the mRNA transcribed by the targetgene. In one embodiment the high density array may contain every probeof a particular length that is complementary to a particular mRNA. Theprobes of the high density array are then hybridized with their targetnucleic acid alone and then hybridized with a high complexity, highconcentration nucleic acid sample that does not contain the targetscomplementary to the probes. Thus, for example, where the target nucleicacid is an RNA, the probes are first hybridized with their targetnucleic acid alone and then hybridized with RNA made from a cDNA library(e.g., reverse transcribed polyA⁺ mRNA) where the sense of thehybridized RNA is opposite that of the target nucleic acid (to insurethat the high complexity sample does not contain targets for theprobes). Those probes that show a strong hybridization signal with theirtarget and little or no cross-hybridization with the high complexitysample are preferred probes for use in the high density arrays of thisinvention.

[0108] The high density array may additionally contain mismatch controlsfor each of the probes to be tested. In a preferred embodiment, themismatch controls contain a central mismatch. Where both the mismatchcontrol and the target probe show high levels of hybridization (e.g.,the hybridization to the mismatch is nearly equal to or greater than thehybridization to the corresponding test probe), the test probe ispreferably not used in the high density array.

[0109] In a particularly preferred embodiment, optimal probes areselected according to the following method: First, as indicated above,an array is provided containing a multiplicity of oligonucleotide probescomplementary to subsequences of the target nucleic acid. Theoligonucleotide probes may be of a single length or may span a varietyof lengths ranging from 5 to 50 nucleotides. The high density array maycontain every probe of a particular length that is complementary to aparticular mRNA or may contain probes selected from various regions ofparticular mRNAs. For each target-specific probe the array also containsa mismatch control probe; preferably a central mismatch control probe.

[0110] The oligonucleotide array is hybridized to a sample containingtarget nucleic acids having subsequences complementary to theoligonucleotide probes and the difference in hybridization intensitybetween each probe and its mismatch control is determined. Only thoseprobes where the difference between the probe and its mismatch controlexceeds a threshold hybridization intensity (e.g. preferably greaterthan 10% of the background signal intensity, more preferably greaterthan 20% of the background signal intensity and most preferably greaterthan 50% of the background signal intensity) are selected. Thus, onlyprobes that show a strong signal compared to their mismatch control areselected.

[0111] The probe optimization procedure can optionally include a secondround of selection. In this selection, the oligonucleotide probe arrayis hybridized with a nucleic acid sample that is not expected to containsequences complementary to the probes. Thus, for example, where theprobes are complementary to the RNA sense strand a sample of antisenseRNA is provided. Of course, other samples could be provided such assamples from organisms or cell lines known to be lacking a particulargene, or known for not expressing a particular gene.

[0112] Only those probes where both the probe and its mismatch controlshow hybridization intensities below a threshold value (e.g. less thanabout 5 times the background signal intensity, preferably equal to orless than about 2 times the background signal intensity, more preferablyequal to or less than about 1 times the background signal intensity, andmost preferably equal or less than about half background signalintensity) are selected. In this way probes that show minimalnon-specific binding are selected. Finally, in a preferred embodiment,the n probes (where n is the number of probes desired for each targetgene) that pass both selection criteria and have the highesthybridization intensity for each target gene are selected forincorporation into the array, or where already present in the array, forsubsequent data analysis. Of course, one of skill in the art, willappreciate that either selection criterion could be used alone forselection of probes.

[0113] III. Synthesis of High Density Arrays

[0114] Methods of forming high density arrays of oligonucleotides,peptides and other polymer sequences with a minimal number of syntheticsteps are known. The oligonucleotide analogue array can be synthesizedon a solid substrate by a variety of methods, including, but not limitedto, light-directed chemical coupling, and mechanically directedcoupling. See Pirrung et al., U.S. Pat. No. 5,143,854 (see also PCTApplication No. WO 90/15070) and Fodor et al., PCT Publication Nos. WO92/10092 and WO 93/09668 which disclose methods of forming vast arraysof peptides, oligonucleotides and other molecules using, for example,light-directed synthesis techniques. See also, Fodor et al., Science,251, 767-77 (1991). These procedures for synthesis of polymer arrays arenow referred to as VLSIPS™ procedures. Using the VLSIPS™ approach, oneheterogenous array of polymers is converted, through simultaneouscoupling at a number of reaction sites, into a different heterogenousarray. See, U.S. application Ser. Nos. 07/796,243 and 07/980,523.

[0115] The development of VLSIPS™ technology as described in theabove-noted U.S. Pat. No. 5,143,854 and PCT patent publication Nos. WO90/15070 and 92/10092, is considered pioneering technology in the fieldsof combinatorial synthesis and screening of combinatorial libraries.More recently, patent application Ser. No. 08/082,937, filed Jun. 25,1993 describes methods for making arrays of oligonucleotide probes thatcan be used to check or determine a partial or complete sequence of atarget nucleic acid and to detect the presence of a nucleic acidcontaining a specific oligonucleotide sequence.

[0116] In brief, the light-directed combinatorial synthesis ofoligonucleotide arrays on a glass surface proceeds using automatedphosphoramidite chemistry and chip masking techniques. In one specificimplementation, a glass surface is derivatized with a silane reagentcontaining a functional group, e.g., a hydroxyl or amine group blockedby a photolabile protecting group. Photolysis through a photolithogaphicmask is used selectively to expose functional groups which are thenready to react with incoming 5′-photoprotected nucleosidephosphoramidites. The phosphoramidites react only with those sites whichare illuminated (and thus exposed by removal of the photolabile blockinggroup). Thus, the phosphoramidites only add to those areas selectivelyexposed from the preceding step. These steps are repeated until thedesired array of sequences have been synthesized on the solid surface.Combinatorial synthesis of different oligonucleotide analogues atdifferent locations on the array is determined by the pattern ofillumination during synthesis and the order of addition of couplingreagents.

[0117] In the event that an oligonucleotide analogue with a polyamidebackbone is used in the VLSIPS™ procedure, it is generally inappropriateto use phosphoramidite chemistry to perform the synthetic steps, sincethe monomers do not attach to one another via a phosphate linkage.Instead, peptide synthetic methods are substituted. See, e.g., Pirrunget al. U.S. Pat. No. 5,143,854.

[0118] Peptide nucleic acids are commercially available from, e.g.,Biosearch, Inc. (Bedford, Mass.) which comprise a polyamide backbone andthe bases found in naturally occurring nucleosides. Peptide nucleicacids are capable of binding to nucleic acids with high specificity, andare considered “oligonucleotide analogues” for purposes of thisdisclosure.

[0119] In addition to the foregoing, additional methods which can beused to generate an array of oligonucleotides on a single substrate aredescribed in co-pending application Ser. No. 07/980,523, filed Nov. 20,1992, and Ser. No. 07/796,243, filed Nov. 22, 1991 and in PCTPublication No. WO 93/09668. In the methods disclosed in theseapplications, reagents are delivered to the substrate by either (1)flowing within a channel defined on predefined regions or (2) “spotting”on predefined regions. However, other approaches, as well ascombinations of spotting and flowing, may be employed. In each instance,certain activated regions of the substrate are mechanically separatedfrom other regions when the monomer solutions are delivered to thevarious reaction sites.

[0120] A typical “flow channel” method applied to the compounds andlibraries of the present invention can generally be described asfollows. Diverse polymer sequences are synthesized at selected regionsof a substrate or solid support by forming flow channels on a surface ofthe substrate through which appropriate reagents flow or in whichappropriate reagents are placed. For example, assume a monomer “A” is tobe bound to the substrate in a first group of selected regions. Ifnecessary, all or part of the surface of the substrate in all or a partof the selected regions is activated for binding by, for example,flowing appropriate reagents through all or some of the channels, or bywashing the entire substrate with appropriate reagents. After placementof a channel block on the surface of the substrate, a reagent having themonomer A flows through or is placed in all or some of the channel(s).The channels provide fluid contact to the first selected regions,thereby binding the monomer A on the substrate directly or indirectly(via a spacer) in the first selected regions.

[0121] Thereafter, a monomer B is coupled to second selected regions,some of which may be included among the first selected regions. Thesecond selected regions will be in fluid contact with a second flowchannel(s) through translation, rotation, or replacement of the channelblock on the surface of the substrate; through opening or closing aselected valve; or through deposition of a layer of chemical orphotoresist. If necessary, a step is performed for activating at leastthe second regions. Thereafter, the monomer B is flowed through orplaced in the second flow channel(s), binding monomer B at the secondselected locations. In this particular example, the resulting sequencesbound to the substrate at this stage of processing will be, for example,A, B, and AB. The process is repeated to form a vast array of sequencesof desired length at known locations on the substrate.

[0122] After the substrate is activated, monomer A can be flowed throughsome of the channels, monomer B can be flowed through other channels, amonomer C can be flowed through still other channels, etc. In thismanner, many or all of the reaction regions are reacted with a monomerbefore the channel block must be moved or the substrate must be washedand/or reactivated. By making use of many or all of the availablereaction regions simultaneously, the number of washing and activationsteps can be minimized.

[0123] One of skill in the art will recognize that there are alternativemethods of forming channels or otherwise protecting a portion of thesurface of the substrate. For example, according to some embodiments, aprotective coating such as a hydrophilic or hydrophobic coating(depending upon the nature of the solvent) is utilized over portions ofthe substrate to be protected, sometimes in combination with materialsthat facilitate wetting by the reactant solution in other regions. Inthis manner, the flowing solutions are further prevented from passingoutside of their designated flow paths.

[0124] The “spotting” methods of preparing compounds and libraries ofthe present invention can be implemented in much the same manner as theflow channel methods. For example, a monomer A can be delivered to andcoupled with a first group of reaction regions which have beenappropriately activated. Thereafter, a monomer B can be delivered to andreacted with a second group of activated reaction regions. Unlike theflow channel embodiments described above, reactants are delivered bydirectly depositing (rather than flowing) relatively small quantities ofthem in selected regions. In some steps, of course, the entire substratesurface can be sprayed or otherwise coated with a solution. In preferredembodiments, a dispenser moves from region to region, depositing only asmuch monomer as necessary at each stop. Typical dispensers include amicropipette to deliver the monomer solution to the substrate and arobotic system to control the position of the micropipette with respectto the substrate. In other embodiments, the dispenser includes a seriesof tubes, a manifold, an array of pipettes, or the like so that variousreagents can-be delivered to the reaction regions simultaneously.

[0125] IV. Hybridization.

[0126] Nucleic acid hybridization simply involves providing a denaturedprobe and target nucleic acid under conditions where the probe and itscomplementary target can form stable hybrid duplexes throughcomplementary base pairing. The nucleic acids that do not form hybridduplexes are then washed away leaving the hybridized nucleic acids to bedetected, typically through detection of an attached detectable label.It is generally recognized that nucleic acids are denatured byincreasing the temperature or decreasing the salt concentration of thebuffer containing the nucleic acids. Under low stringency conditions(e.g., low temperature and/or high salt) hybrid duplexes (e.g., DNA:DNA,RNA:RNA, or RNA:DNA) will form even where the annealed sequences are notperfectly complementary. Thus specificity of hybridization is reduced atlower stringency. Conversely, at higher stringency (e.g., highertemperature or lower salt) successful hybridization requires fewermismatches.

[0127] One of skill in the art will appreciate that hybridizationconditions may be selected to provide any degree of stringency. In apreferred embodiment, hybridization is performed at low stringency inthis case in 6×SSPE-T at 37° C. (0.005% Triton X-100) to ensurehybridization and then subsequent washes are performed at higherstringency (e.g., 1×SSPE-T at 37° C.) to eliminate mismatched hybridduplexes. Successive washes may be performed at increasingly higherstringency (e.g., down to as low as 0.25×SSPE-T at 37° C. to 50° C.)until a desired level of hybridization specificity is obtained.Stringency can also be increased by addition of agents such asformamide. Hybridization specificity may be evaluated by comparison ofhybridization to the test probes with hybridization to the variouscontrols that can be present (e.g., expression level control,normalization control, mismatch controls, etc.).

[0128] In general, there is a tradeoff between hybridization specificity(stringency) and signal intensity. Thus, in a preferred embodiment, thewash is performed at the highest stringency that produces consistentresults and that provides a signal intensity greater than approximately10% of the background intensity. Thus, in a preferred embodiment, thehybridized array may be washed at successively higher stringencysolutions and read between each wash. Analysis of the data sets thusproduced will reveal a wash stringency above which the hybridizationpattern is not appreciably altered and which provides adequate signalfor the particular oligonucleotide probes of interest.

[0129] In a preferred embodiment, background signal is reduced by theuse of a detergent (e.g., C-TAB) or a blocking reagent (e.g., sperm DNA,cot-1 DNA, etc.) during the hybridization to reduce non-specificbinding. In a particularly preferred embodiment, the hybridization isperformed in the presence of about 0.5 mg/ml DNA (e.g., herring spermDNA). The use of blocking agents in hybridization is well known to thoseof skill in the art (see, e.g., Chapter 8 in P. Tijssen, supra.)

[0130] The stability of duplexes formed between RNAs or DNAs aregenerally in the order of RNA:RNA>RNA:DNA>DNA:DNA, in solution. Longprobes have better duplex stability with a target, but poorer mismatchdiscrimination than shorter probes (mismatch discrimination refers tothe measured hybridization signal ratio between a perfect match probeand a single base mismatch probe). Shorter probes (e.g., 8-mers)discriminate mismatches very well, but the overall duplex stability islow.

[0131] Altering the thermal stability (T_(m)) of the duplex formedbetween the target and the probe using, e.g., known oligonucleotideanalogues allows for optimization of duplex stability and mismatchdiscrimination. One useful aspect of altering the T_(m) arises from thefact that adenine-thymine (A-T) duplexes have a lower T_(m) thanguanine-cytosine (G-C) duplexes due in part to the fact that the A-Tduplexes have 2 hydrogen bonds per base-pair, while the G-C duplexeshave 3 hydrogen bonds per base pair. In heterogeneous oligonucleotidearrays in which there is a non-uniform distribution of bases, it is notgenerally possible to optimize hybridization for each oligonucleotideprobe simultaneously. Thus, in some embodiments, it is desirable toselectively destabilize G-C duplexes and/or to increase the stability ofA-T duplexes. This can be accomplished, e.g., by substituting guanineresidues in the probes of an array which form G-C duplexes withhypoxanthine, or by substituting adenine residues in probes which formA-T duplexes with 2,6 diaminopurine or by using the salt tetramethylammonium chloride (TMACl) in place of NaCl.

[0132] Altered duplex stability conferred by using oligonucleotideanalogue probes can be ascertained by following, e.g., fluorescencesignal intensity of oligonucleotide analogue arrays hybridized with atarget oligonucleotide over time. The data allow optimization ofspecific hybridization conditions at, e.g., room temperature (forsimplified diagnostic applications in the future).

[0133] Another way of verifying altered duplex stability is by followingthe signal intensity generated upon hybridization with time. Previousexperiments using DNA targets and DNA chips have shown that signalintensity increases with time, and that the more stable duplexesgenerate higher signal intensities faster than less stable duplexes. Thesignals reach a plateau or “saturate” after a certain amount of time dueto all of the binding sites becoming occupied. These data allow foroptimization of hybridization, and determination of the best conditionsat a specified temperature.

[0134] Methods of optimizing hybridization conditions are well known tothose of skill in the art (see, e.g., Laboratory Techniques inBiochemistry and Molecular Biology, Vol. 24: Hybridization With NucleicAcid Probes, P. Tijssen, ed. Elsevier, N.Y., (1993)).

[0135] V. Signal Detection.

[0136] Means of detecting labeled target (sample) nucleic acidshybridized to the probes of the high density array are known to those ofskill in the art. Thus, for example, where a colorimetric label is used,simple visualization of the label is sufficient. Where a radioactivelabeled probe is used, detection of the radiation (e.g with photographicfilm or a solid state detector) is sufficient.

[0137] In a preferred embodiment, however, the target nucleic acids arelabeled with a fluorescent label and the localization of the label onthe probe array is accomplished with fluorescent microscopy. Thehybridized array is excited with a light source at the excitationwavelength of the particular fluorescent label and the resultingfluorescence at the emission wavelength is detected. In a particularlypreferred embodiment, the excitation light source is a laser appropriatefor the excitation of the fluorescent label.

[0138] The confocal microscope may be automated with acomputer-controlled stage to automatically scan the entire high densityarray. Similarly, the microscope may be equipped with a phototransducer(e.g., a photomultiplier, a solid state array, a ccd camera, etc.)attached to an automated data acquisition system to automatically recordthe fluorescence signal produced by hybridization to eacholigonucleotide probe on the array. Such automated systems are describedat length in U.S. Pat. No. 5,143,854, PCT Application 20 92/10092, andcopending U.S. Ser. No. 08/195,889 filed on Feb. 10, 1994. Use of laserillumination in conjunction with automated confocal microscopy forsignal detection permits detection at a resolution of better than about100 μm, more preferably better than about 50 μm, and most preferablybetter than about 25 μm.

[0139] VI. Signal Evaluation.

[0140] One of skill in the art will appreciate that methods forevaluating the hybridization results vary with the nature of thespecific probe nucleic acids used as well as the controls provided. Inthe simplest embodiment, simple quantification of the fluorescenceintensity for each probe is determined. This is accomplished simply bymeasuring probe signal strength at each location (representing adifferent probe) on the high density array (e.g., where the label is afluorescent label, detection of the amount of florescence (intensity)produced by a fixed excitation illumination at each location on thearray). Comparison of the absolute intensities of an array hybridized tonucleic acids from a “test” sample with intensities produced by a“control” sample provides a measure of the relative expression of thenucleic acids that hybridize to each of the probes.

[0141] One of skill in the art, however, will appreciate thathybridization signals will vary in strength with efficiency ofhybridization, the amount of label on the sample nucleic acid and theamount of the particular nucleic acid in the sample. Typically nucleicacids present at very low levels (e.g., <1 pM) will show a very weaksignal. At some low level of concentration, the signal becomes virtuallyindistinguishable from background. In evaluating the hybridization data,a threshold intensity value may be selected below which a signal is notcounted as being essentially indistinguishable from background.

[0142] Where it is desirable to detect nucleic acids expressed at lowerlevels, a lower threshold is chosen. Conversely, where only highexpression levels are to be evaluated a higher threshold level isselected. In a preferred embodiment, a suitable threshold is about 10%above that of the average background signal.

[0143] In addition, the provision of appropriate controls permits a moredetailed analysis that controls for variations in hybridizationconditions, cell health, non-specific binding and the like. Thus, forexample, in a preferred embodiment, the hybridization array is providedwith normalization controls as described above in Section II.A.2. Thesenormalization controls are probes complementary to control sequencesadded in a known concentration to the sample. Where the overallhybridization conditions are poor, the normalization controls will showa smaller signal reflecting reduced hybridization. Conversely, wherehybridization conditions are good, the normalization controls willprovide a higher signal reflecting the improved hybridization.Normalization of the signal derived from other probes in the array tothe normalization controls thus provides a control for variations inhybridization conditions. Typically, normalization is accomplished bydividing the measured signal from the other probes in the array by theaverage signal produced by the normalization controls. Normalization mayalso include correction for variations due to sample preparation andamplification. Such normalization may be accomplished by dividing themeasured signal by the average signal from the samplepreparation/amplfication control probes (e.g., the Bio B probes). Theresulting values may be multiplied by a constant value to scale theresults.

[0144] As indicated above, the high density array can include mismatchcontrols. In a preferred embodiment, there is a mismatch control havinga central mismatch for every probe (except the normalization controls)in the array. It is expected that after washing in stringent conditions,where a perfect match would be expected to hybridize to the probe, butnot to the mismatch, the signal from the mismatch controls should onlyreflect non-specific binding or the presence in the sample of a nucleicacid that hybridizes with the mismatch. Where both the probe in questionand its corresponding mismatch control both show high signals, or themismatch shows a higher signal than its corresponding test probe, thereis a problem with the hybridization and the signal from those probes isignored. The difference in hybridization signal intensity between thetarget specific probe and its corresponding mismatch control is ameasure of the discrimination of the target-specific probe. Thus, in apreferred embodiment, the signal of the mismatch probe is subtractedfrom the signal from its corresponding test probe to provide a measureof the signal due to specific binding of the test probe.

[0145] The concentration of a particular sequence can then be determinedby measuring the signal intensity of each of the probes that bindspecifically to that gene and normalizing to the normalization controls.Where the signal from the probes is greater than the mismatch, themismatch is subtracted. Where the mismatch intensity is equal to orgreater than its corresponding test probe, the signal is ignored. Theexpression level of a particular gene can then be scored by the numberof positive signals (either absolute or above a threshold value), theintensity of the positive signals (either absolute or above a selectedthreshold value), or a combination of both metrics (e.g., a weightedaverage).

[0146] It is a surprising discovery of this invention, thatnormalization controls are often unnecessary for useful quantificationof a hybridization signal. Thus, where optimal probes have beenidentified in the two step selection process as described above, inSection II.B., the average hybridization signal produced by the selectedoptimal probes provides a good quantified measure of the concentrationof hybridized nucleic acid.

[0147] VI. Monitoring Expression Levels

[0148] As indicated above, the methods of this invention may be used tomonitor expression levels of a gene in a wide variety of contexts. Forexample, where the effects of a drug on gene expression is to bedetermined the drug will be administered to an organism, a tissuesample, or a cell. Nucleic acids from the tissue sample, cell, or abiological sample from the organism and from an untreated organismtissue sample or cell are isolated as described above, hybridized to ahigh density probe array containing probes directed to the gene ofinterest and the expression levels of that gene are determined asdescribed above.

[0149] Similarly, where the expression levels of a disease marker (e.g.,P53, RTK, or HER2) are to be detected (e.g., for the diagnosis of apathological condition in a patient), comparison of the expressionlevels of the disease marker in the sample to disease markers from ahealthy organism will reveal any deviations in the expression levels ofthe marker in the test sample as compared to the healthy sample.Correlation of such deviations with a pathological condition provides adiagnostic assay for that condition.

EXAMPLES

[0150] The following examples are offered to illustrate, but not tolimit the present invention.

Example 1 Detection of the Expression Levels of Target Genes.

[0151] Experiments were designed to evalutate the specificity ofhybridization, the relationship between hybridization signal andconcentration of target nucleic acid, and the quantifiability of RNAdetection at low concentration levels. These experiments involvedhybridizing labeled RNA from a number of preselected genes (IL-2, IL-3,IL4, IL-6, IL-10, IL-12p40, GM-CSF, IFN-γ, TNF-α, mCTLA8, β-actin,GAPDH, IL-11 receptor, and Bio B) to a high density oligonucleotideprobe array comprising a large number of probes complementary tosubsequences of these genes (see, Section B, below for a description ofthe array) in the presence or absence of an RNA sample transcribed froma cDNA library. The target genes were hybridized to the high densityprobe array either individually, together, or individually or togetherin the presence of labeled RNA transcribed from a murine cDNA library asdescribed below.

[0152] A) Preparation of Labeled RNA.

[0153] 1) From Each of the Preselected Genes.

[0154] Fourteen genes (IL-2, IL-3, IL-4, IL-6, II-10, IL-12p40, GM-CSF,IFN-γ, TNF-α, CTLA8, β-actin, GAPDH, IL-11 receptor, and Bio B) wereeach cloned into the p Bluescript II KS (+) phagemid (Stratagene, LaJolla, Calif., USA). The orientation of the insert was such that T3 RNApolymerase gave sense transcripts and T7 polymerase gave antisense RNA.

[0155] In vitro transcription was done with cut templates in a mannerlike that described by Melton et al., Nucleic Acids Research, 12:7035-7056 (1984). A typical in vitro transcription reaction used 5 μgDNA template, a buffer such as that included in Ambion's Maxiscript invitro Transcription Kit (Ambion Inc., Huston, Tex., USA) and GTP (3 mM),ATP (1.5 mM), UTP and fluoresceinated UTP (3 mM total, UTP: F1-UTP 1:1)and CTP and fluoresceinated CTP (2 mM total, CTP: F1-CTP, 3:1).Reactions done in the Ambion buffer had 20 mM DTT and RNase inhibitor.The T7 polymerase was a high concentration polymerase (activity about2500 units/μL) available from Epicentre Technologies, Madison, Wis.,USA. The reaction was run from 1.5 to about 8 hours.

[0156] The nucleotide triphosphates were removed using a microcon-100 orPharmacia microspin S-200 column. The labeled RNA was then fragmented ina pH 8.1 Tris-HCl buffer containing 30 mM Mg(OAc)₂ at 94° C. for 30 to40 minutes depending on the length of the RNA transcript.

[0157] 2) From cDNA Libraries.

[0158] Labeled RNA was produced from one of two murine cell lines; T10,a B cell plasmacytoma which was known not to express the genes (exceptIL-10, actin and GAPDH) used as target genes in this study, and 2D6, anIL-12 growth dependent T cell line (Th₁ subtype) that is known toexpress most of the genes used as target genes in this study. Thus, RNAderived from the T10 cell line provided a good total RNA baselinemixture suitable for spiking with known quantities of RNA from theparticular target genes. In contrast, mRNA derived from the 2D6 cellline provided a good positive control providing typical endogenouslytranscribed amounts of the RNA from the target genes, To produce the T10cDNA library, cDNA was directionally cloned into λSHlox-1 (GibcoBRL,Gaithersburg, Md., USA) at EcoRi/HInd III to give a phage library. Thephage library was converted to a plasmid library using “automaticCre-loxP plasmid subcloning according to the method of Palazzolo, etal., Gene, 88: 25-36 (1990). After this the DNA was linearized with NotI and T7 polymerase was used to generate labeled T10 RNA in an in vitrotranscription reaction as described above.

[0159] Labeled 2D6 mRNA was produced by directionally cloning the 2D6cDNA with αZipLox, NotI-SalI arms available from GibcoBRL in a mannersimilar to T10. The linearized pZ11 library was transcribed with T7 togenerate sense RNA as described above.

[0160] B) High Density Array Preparation

[0161] A high density array of 20 mer oligonucleotide probes wasproduced using VLSIPS technology. The high density array included theoligonucleotide probes as listed in Table 1. A central mismatch controlprobe was provided for each gene-specific probe resulting in a highdensity array containing over 16,000 different oligonucleotide probes.TABLE 1 High density array design. For every probe there was also amismatch control having a central 1 base mismatch. Probe Type TargetNucleic Acid Number of Probes Test Probes: IL-2 691 IL-3 751 IL-4 361IL-6 691 IL-10 481 IL-12p40 911 GM-CSF 661 IFN-γ 991 TNF-α 641 mCTLA8391 IL-11 receptor 158 House Keeping Genes: GAPDH 388 β-actin 669Bacterial gene (sample Bio B 286 preparation/amplification control)

[0162] The high density array was synthesized on a planar glass slide.

[0163] C) Hybridization Conditions.

[0164] The RNA transcribed from cDNA was then hybridized to the highdensity oligonucleotide probe array at low stringency (e.g., in 6×SSPE-Twith 0.5 mg/ml unlabeled, degraded herring sperm DNA as a blockingagent, at 37° C. for 18 hours). The hybridized arrays were washed underprogressively more stringent conditions, (e.g., in 1×SSPE-T at 37° C.for 7 minutes down to 0.25×SSPE-T overnight) with the hybridized arraybeing read by a laser-illuminated scanning confocal fluorescencemicroscope between washes.

[0165] It was discovered that the excess RNA in the sample frequentlybound up the high density array probes and/or targets and apparentlyprevented the probes from specifically binding with their intendedtarget. This problem was obviated by hybridizing at temperatures over30° C. and/or adding CTAB (cetyltrimethylammonium bromide) a detergent.

[0166] D) Optimization of Probe Selection

[0167] In order to optimize probe selection for each of the targetgenes, the high density array of oligonucleotide probes was hybridizedwith the mixture of labeled RNAs transcribed from each of the targetgenes. Fluorescence intensity at each location on the high density arraywas determined by scanning the high density array with a laserilluminated scanning confocal fluorescence microscope connected to adata acquisition system.

[0168] Probes were then selected for further data analysis in a two-stepprocedure. First, in order to be counted, the difference in intensitybetween a probe and its corresponding mismatch probe had to exceed athreshold limit (50 counts, or about half background, in this case).This eliminated from consideration probes that did not hybridize welland probes for which the mismatch control hybridizes at an intensitycomparable to the perfect match.

[0169] The high density array was hybridized to a labeled RNA samplewhich, in principle, contains none of the sequences on the high densityarray. In this case, the oligonucleotide probes were chosen to becomplementary to the sense RNA. Thus, an anti-sense RNA populationshould have been incapable of hybridizing to any of the probes on thearray. Where either a probe or its mismatch showed a signal above athreshold value (100 counts above background) it was not included insubsequent analysis.

[0170] Then, the signal for a particular gene was counted as the averagedifference (perfect match—mismatch control) for the selected probes foreach gene.

[0171] D) Interpretation of Results.

[0172] 1) Specificity of Hybridization

[0173] In order to evaluate the specificity of hybridization, the highdensity array described above was hybridized with 50 pM of the RNA sensestrand of IL-2, IL-3, IL-4, IL-6, Actin, GAPDH and Bio B or IL-10,IL-12p40, GM-CSF, IFN-γ, TNF-α, mCTLA8 and Bio B. The hybridized arrayshowed strong specific signals for each of the test target nucleic acidswith minimal cross hybridization.

[0174] 2) Relationship between Target Concentration and HybridizationSignal

[0175] In order to evaluate the relationship between hybridizationsignal and target probe concentration, hybridization intensity wasmeasured as a function of concentration of the RNAs for one or more ofthe target genes. FIG. 1 shows the results of this experiment. Graphs Aand B are plots of the hybridization intensity of high concentrations(50 pM to 10 nM) of IL-4 hybridized to the array for 90 minutes at 22°C. Plot B merely expands the ordinate of plot A to show the lowconcentration values. In both plots, the hybridization signal increaseswith target concentration and the signal level is proportional to theRNA concentration between 50 pM and 1 nM.

[0176] Graphs C and D are plots of the average hybridization intensitydifferences of the 1000 most intense probes when the array ishybridized, for 15 hours at 37° C., to a mixture of 0.5 pM to 20 pM eachof labeled RNA from IL-2, IL-3, IL4, IL-6, IL-10, GM-CSF, IFN-γ, TNF-α,mCTLA8, β-actin, GAPDH, and Bio B. Even a signal, in effect, averagedacross 13 different target RNAs, shows an intensity proportional totarget RNA concentration. Again, Graph D expands the ordinate of plot Ato show the low concentration signal.

[0177] At high target nucleic acid concentration, the hybridization timecould be decreased, while at lower target nucleic acid concentration,the hybridization time should be increased. By varying hybridizationtime, it is possible to obtain a substantially linear relationshipbetween target RNA concentration and hybridization intensity for a widerange of target RNA concentrations.

[0178] 3) Detection of Gene Expression Levels in a Complex TargetSample.

[0179] In order to evaluate the ability of the high density arraydescribed above to measure variations in expression levels of the targetgenes, hybridization was performed with the T10 murine library RNA, thelibrary spiked with 10 pM each of mCTLA8, IL-6, IL-3, IFN-γ, and IL-12and 50 pM of each of these RNA transcripts prepared as described above.

[0180] Because simply spiking the RNA mixture with the selected targetgenes and then immediately hybridizing might provide an artificiallyelevated reading relative to the rest of the mixture, the spiked samplewas treated to a series of procedures to mitigate differences betweenthe library RNA and the added RNA. Thus the “spike” was added to thesample which was then heated to 37° C. and annealed. The sample was thenfrozen, thawed, boiled for 5 minutes, cooled on ice and allowed toreturn to room temperature before performing the hybridization.

[0181] The sample was then hybridized at low stringency and washed atprogressively higher stringency as described above. The best probes foreach target gene were selected as described above, in Section D, and theaverage intensity of the difference (perfect match—mismatch) of theprobes for each target gene is plotted in FIGS. 2 and 3.

[0182] A 50 pM spike represents a target mRNA concentration of about 1in 24,000, while a 10 pM spike represents a target mRNA concentration ofabout 1 in 120,000. As illustrated in FIG. 2, the high density arrayeasily resolves and quantifies the relative expression levels of each ofthe target genes in one simultaneous hybridization. Moreover, therelative expression level is quantifiable with a 5 fold difference inconcentration of the target mRNA resulting in a 3 to 6 fold differencein hybridization intensity for the five spiked targets.

[0183]FIG. 3 replots FIG. 2 on a condensed scale so that the expressionlevels of constitutively expressed GAPDH and Actin and the level ofIL-10 which is endogenously expressed by the cell line, is visible. Itis notable that the single hybridization to the array resolvedexpression levels varying from 1 in 1000 for GAPDH to 1 in 124,000 forthe spiked mRNAs without the high concentration RNA (the RNA library)overwhelming the signal from the genes expressed at low levels (e.g.,IL-10).

[0184] It is also worthy of note that the endogenous (intrinsic) IL-10was transcribed at a level comparable to or lower than the spiked RNAs(see FIG. 2) and the method thus is capable of quantifying the levels oftranscription of genes that are transcribed at physiologically realisticlevels.

[0185] The method described herein thus easily quantifies changes in RNAconcentrations of 5 to 10 fold. Detection is highly specific andquantitative at levels as low as 1 in 120,000. The sensitivity andspecificity is sufficient to detect low concentration RNAs (comparableto about 20 to 30 per cell) in the presence of total mammalian cellmessage populations. Other experiments have detected concentrations aslow as 1 in 300,000, comparable to about 10 RNAs per cell and the methodclearly provides a means for simultaneously screening transcriptionlevels of literally hundreds of genes simultaneously in a complex RNApool.

[0186] It is understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication and scope of the appended claims. All publications, patents,and patent applications cited herein are hereby incorporated byreference for all purposes.

What is claimed is:
 1. A method of simultaneously monitoring theexpression of a multiplicity of genes, said method comprising: (a)providing a pool of target nucleic acids comprising RNA transcripts ofone or more of said genes, or nucleic acids derived from said RNAtranscripts; (b) hybridizing said pool of nucleic acids to an array ofoligonucleotide probes immobilized on a surface, said array comprisingmore than 100 different oligonucleotides wherein each differentoligonucleotide is localized in a predetermined region of said surface,the density of said different oligonucleotides is greater than about 60different oligonucleotides per 1 cm², and said olignucleotide probes arecomplementary to said RNA transcripts or said nucleic acids derived fromsaid RNA transcripts; and (c) quantifying the hybridization of saidnucleic acids to said array.
 2. The method of claim 1, wherein theconcentration of said RNA transcripts in said pool, or nucleic acidsderived from said RNA transcripts, is proportional to the expressionlevels of said genes.
 3. The method of claim 1, wherein said array ofoligonucleotides further comprises mismatch control probes.
 4. Themethod of claim 3, wherein said quantifying comprises calculating thedifference in hybridization signal intensity between each of saidoligonucleotide probes and its corresponding mismatch control probe. 5.The method of claim 4, wherein said quantifying comprises calculatingthe average difference in hybridization signal intensity between each ofsaid oligonucleotide probes and its corresponding mismatch control probefor each gene.
 6. The method of claim 3, wherein said oligonucleotideprobes present in said array are oligonucleotide probes selectedaccording to the method of claim
 29. 7. The method of claim 3, whereinsaid oligonucleotide probes present in said array are oligonucleotideprobes selected according to the method of claim
 29. 8. The method ofclaim 3, wherein said oligonucleotide probes analyzed in saidquantifying step are selected according to the method of claim
 29. 9.The method of claim 3, wherein said oligonucleotide probes analyzed insaid quantifying step are selected according to the method of claim 29.10. The method of claim 1, wherein hybridization and quantification isaccomplished in under 48 hours.
 11. The method of claim 1, wherein saidmultiplicity of genes is 100 genes or more.
 12. The method of claim 1,wherein for each gene, said array comprises at least 10 differentoligonucleotide probes complementary to subsequences of that gene. 13.The method of claim 1, wherein said hybridization is performed with afluid volume of about 250 μl or less.
 14. The method of claim 1, whereinsaid quantifying comprises detecting a hybridization signal that isproportional to the concentration of said RNA in said nucleic acidsample.
 15. The method of claim 1, wherein said oligonucleotides arefrom 5 to about 50 nucleotides in length.
 16. The method of claim 1,wherein said oligonucleotides are synthesized by light-directed polymersynthesis.
 17. The method of claim 1, wherein said array comprisesoligonucleotide sequences from constitutively expressed control genes.18. The method of claim 17, wherein said control genes are selected fromthe group consisting of β-actin, GAPDH, and the transferrin receptor.19. The method of claim 1, wherein said hybridization comprises ahybridization at low stringency of 30° C. to 50° C. and 6×SSPE-T orlower and a wash at higher stringency.
 20. The method of claim 1,wherein said pool of nucleic acids is a pool of mRNAs.
 21. The method ofclaim 1, wherein said pool of nucleic acids is a pool of RNAs in vitrotranscribed from a pool of cDNAs.
 22. The method of claim 1, whereinsaid pool of nucleic acids is amplified from a biological sample. 23.The method of claim 1, wherein said pool of nucleic acids comprisesfluorescently labeled nucleic acids.
 24. The method of claim 1, whereinsaid detecting comprises quantifying fluorescence of a label on saidhybridized nucleic acids at a spatial resolution of 100 μm or higher.25. The method of claim 24, wherein said quantification is by means of ascanning confocal fluorescence microscope.
 26. The method of claim 1,wherein said providing comprises: (a) hybridizing a pool of RNAs with apool of oligonucleotides having the same sequences as saidoligonucleotide probes to form a pool of hybridized nucleic acids; (b)treating said pool of hybridized nucleic acids with RNase A, therebydigesting single stranded nucleic acid sequences and leaving intact thehybridized double stranded regions; (c) denaturing the hybridizeddouble-stranded regions and removing said oligonucleotides therebyleaving a pool of RNAs enhanced for those RNAs complementary to theoligonucleotide probes in said high density array.
 27. The method ofclaim 1, wherein said providing comprises: (a) hybridizing a pool ofRNAs with paired target specific oligonucleotides where said pairedtarget specific oligonucleotides are complementary to regions flankingsubsequences complementary to said oligonucleotide probes in said highdensity array; (b) treating said pool of nucleic acids with RNase H todigest the hybridized (double stranded) nucleic acid sequences; (c)isolating the remaining nucleic acid sequences having a length aboutequivalent to the region flanked by said paired target specificoligonucleotides.
 28. The method of claim 1, wherein said providingcomprises: (a) hybridizing a pool of polyA⁺ mRNAs with oligonucleotidesthat hybridize specifically with particular preselected mRNA targetmessages; (b) treating said pool of nucleic acids with RNase H to digestthe hybridized (double stranded) nucleic acid sequences therebyseparating the coding sequence from the polyA⁺ tail; (c) isolating oramplifying the remaining polyA⁺ RNA in said pool.
 29. A method ofselecting a set of oligonucleotide probes that specifically bind to atarget nucleic acid, said method comprising: (a) providing a highdensity array of oligonucleotide probes said array comprising amultiplicity of oligonucleotide probes, wherein each probe iscomplementary to a subsequence of said target nucleic acid and for eachprobe there is a corresponding mismatch control probe; and (b)hybridizing said target nucleic acid to said array of oligonucleotideprobes to identify and select those probes where the difference inhybridization signal intensity between each probe and its mismatchcontrol is detectable.
 30. The method of claim 29, further comprising:(c) hybridizing said array to a pool of nucleic acids comprising nucleicacids other than said target nucleic acids; and (d) selecting probeshaving the lowest hybridization signal and where both the probe and itsmismatch control have a hybridization intensity equal to or less than 10times background.
 31. The method of claim 29, wherein saidoligonucleotide probes ranges in length from about 10 to about
 50. 32.The method of claim 29, wherein said oligonucleotide probes are all thesame length.
 33. The method of claim 29, wherein said difference inhybridization intensity between each probe and its mismatch control isat least 10% of the background signal.
 34. The method of claim 29,wherein said multiplicity of probes includes all the probes of a singlelength that are complementary to a subsequence of said target nucleicacid where said probes have a length between about 5 and 50 nucleotides.35. The method of claim 29, wherein said array comprises more than 100different oligonucleotides wherein each different oligonucleotide islocalized in a predetermined region of said surface and the density ofsaid different oligonucleotides is greater than about 60 differentoligonucleotides per 1 cm² of said surface.
 36. The method of claim 29,wherein hybridization is performed with a fluid volume of about 250 μlor less.
 37. The method of claim 29, wherein said target nucleic acid isa nucleic acid derived from a gene.
 38. The method of claim 29, whereinsaid oligonucleotide probes are synthesized by light-directed polymersynthesis.
 39. The method of claim 29, wherein said mismatch controlprobes have a centrally located 1 base mismatch.
 40. The method of claim29, wherein said hybridization comprises hybridization at low stringencyof 30° C. to 50° C. and 6×SSPE-T or lower followed by one or more washesat progressively increasing stringency until a desired level ofhybridization specificity is obtained.
 41. The method of claim 30,wherein pool of nucleic acids is a pool of nucleic acids having a senseopposite that of the nucleic acids to which said oligonucleotide probesare complementary.
 42. A composition comprising: an array ofoligonucleotide probes immobilized on a solid support, said arraycomprising more than 100 different oligonucleotides wherein eachdifferent oligonucleotide is localized in a predetermined region of saidsolid support and the density of said different oligonucleotides isgreater than about 60 different oligonucleotides per 1 cm² of said solidsupport; and said oligonucleotide probes are specifically hybridized toone or more fluorescently labeled nucleic acids forming a fluorescentarray such that the fluorescence of said array is indicative of thetranscription levels of a multiplicity of preselected genes.
 43. Thecomposition of claim 42, wherein said array of oligonucleotidescomprises oligonucleotides that hybridize specifically to a gene, ornucleic acids derived from said gene, whose expression is to bedetected.
 44. The composition of claim 42, wherein said fluorescenceintensity is proportional to the transcription levels of saidmultiplicity of preselected genes in a biological sample.
 45. Thecomposition of claim 43, wherein said array of oligonucleotidescomprises mismatch control probes.
 46. The composition of claim 43,wherein said oligonucleotides are from 5 to about 50 nucleotides inlength.
 47. The composition of claim 43, wherein said oligonucleotidesare synthesized by light-directed polymer synthesis.
 48. The compositionof claim 43, wherein said array comprises expression control probeshaving sequences complementary to one or more constitutively expressedgenes.
 49. The composition of claim 48, wherein said constitutivelyexpressed genes are selected from the group consisting of B-actin,GAPDH, and the transferrin receptor.
 50. The composition of claim 43,wherein said pool of nucleic acids is a pool of mRNAs.
 51. Thecomposition of claim 50, wherein said RNAs are in vitro transcribed froma pool of cDNAs.
 52. A kit for the detection of expression levels of amultiplicity of genes, said kit comprising: an array of immobilizedoligonucleotide probes complementary to subsequences of saidmultiplicity of genes, wherein said array comprises at least 100different oligonucleotide probes and the density of said array isgreater than about 60 different oligonucleotides per 1 cm² of surfacearea of a substrate; and instructions describing the use of said arrayfor the quantification of expression levels of said multiplicity ofgenes.
 53. The kit of claim 17, further comprising fluorescent label forlabeling RNA or DNA that is to be hybridized to the oligonucleotides ofsaid array.
 54. The kit of claim 17, further buffers and reagents forthe hybridization of RNA to the oligonucleotides of said array.